1. Field of the Invention
The present invention generally relates to the art of wireless communications. In particular, the present invention relates to the art of searching for the direct sequence spread spectrum pilot signals of base stations to establish communication between a mobile unit and the base station.
2. Description of Related Art
In wireless communications technology, user data (e.g. speech) is encoded in a radio frequency for transmission and reception between a base station and a mobile unit. The radio spectrum allocated by regulatory authorities for a wireless system is trunked to allow simultaneous use of that spectrum block by multiple units.
The most common form of trunked access is the frequency-division multiple access (FDMA) system. In an FDMA system voice is commonly transmitted through analog modulation but can in principle be digitized and transmitted with digital modulation. In FDMA, the spectrum is divided into frequency channels comprised of distinct portions of the spectrum. The limited frequency channels are allocated to users as needed. However, once a frequency channel is assigned to a user, that frequency channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each frequency channel to one, and the total number of users of the entire system, at any instant, to the number of available frequency channels.
Another common trunking system is the time-division multiple access (TDMA) system. TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with an FDMA structure. In TDMA, data (speech) is digitized and compressed to eliminate redundancy thus decreasing the average amount of bits required to be transmitted and received for the same amount of information. The time line of each of the frequency channels used by the TDMA system is divided into "frames" and each of the users sharing the common channel is assigned a time slot within the frames. Each user then transmits a burst of data during its assigned timeslot and transmits nothing during other times. With the exception of delays required by the bursty data transmission, the TDMA system will appear to each of the users sharing the frequency channel to have provided an entire channel to each user.
The FDMA and TDMA combination technique is used by the GSM (global system for mobile communications) digital cellular system. In GSM, each channel is divided up in time into frames during which eight different users share the channel. A GSM time slot is only 577 .mu.s (micro-seconds), and each users gets to use the channel for 577 .mu.s every 4.615 ms (milli-seconds). 577 .mu.s * 8=4.615 ms.
Yet another method for sharing a common channel between multiple users is the code-division multiple access (CDMA) technique using direct sequence spread spectrum modulation. CDMA is relatively new to the cellular technology and is one of the accepted techniques to be included into the next generation of digital cellular systems in the United States of America (U.S.A.).
As with TDMA, the CDMA systems are typically used in conjunction with a PDMA structure, although this is not required. However, unlike the TDMA system, the CDMA system does not separate the multiple users of a common frequency channel using time slices. Rather, in CDMA, multiple users are separated from each other by superimposing a user-specific high-speed code on the modulation of the data of each user. Because the separating code has the effect of spreading the shared channel of each user's transmission, the CDMA system is often called a "spread spectrum" system.
"Direct sequence" spreading is accomplished by multiplying a narrowband information carrying signal by a much wider band spreading signal. The error coded and digitally modulated data (speech) for each of the shared users of the CDMA channel may typically be 9.6, 14.4, or 19.2 KHz wide. This is spread using a much wider spreading signal which may be 1.2288 MHZ wide. Using the wider spreading signal, a CDMA frequency channel can accommodate many users on code sub-channels. The spreading signal is usually a sequence of pseudo random bits (PN code) and is often called a "spreading code," or "chipping code" because it "spreads" or "chips" the much slower data bits. The PN code is different for differing users, allowing a user to distinguish its code sub-channel from other users' sub-channels on the same frequency channel. The PN sequence may be expressed as c(t), where the chipping function, c(), is a function of time t. The PN sequence is generated using a linear feedback shift register (LFSR) which outputs a random-like sequence of digital ones and zeros. These digital ones and zeros are modulated to -1 and +1 respectively and filtered to give the chipping function c(t). Thus the chipping function has the property that c(t).sup.2 =+1. The PN sequence generated by a N-register LFSR is 2.sup.N- 1 chips long, though a common system artificially inserts a zero to extend the full sequence length to 2.sup.15 =32768 chips. That system has a chip rate of 1.2288 MHz, so that the sequence repeats every 26.666 ms.
In a typical system, each base station maintains a pilot channel with its own identifying spreading code for the mobile units to refer to. A pilot signal is a spread signal with no underlying information modulation, such that the exact waveform is known by both transmitter and receiver, with the exception of the waveform timing. The mobile units use the pilot channel to synchronize themselves with the base station so they can effectively communicate with the base station. When a mobile unit is powered on, the mobile unit initially searches for a pilot channel in an attempt to establish a lock with a base station. This process is called "acquisition." In order to "acquire," or lock on, to a base station, the mobile must align its locally generated version of the PN sequence with the PN sequence of the base station by determining the timing of the transmitted pilot's spreading sequence. The present invention provides for an improved acquisition technique.
At power up, a mobile unit must search for a pilot to synchronize its spreading sequence with that of a base station. The acquisition process is generally described using the system as illustrated by FIG. 1. FIG. 1 is a simplified diagram illustrating the major functions of a system which can implement the acquisition process.
In the simplified model of FIG. 1, the radio signal is received by an antenna 12. The signal at line 14 is a radio frequency signal which is about 800 to 900 MHz for cellular communications. The signal at line 14, S.sub.14, can be expressed as EQU S.sub.14 =d(t)c(t-D.sub.base)cos(t)
where
d(t) is the data (speech in digitized form); PA2 c(t-D.sub.base) is the PN short code at delay D.sub.base which is the base station delay; and PA2 cos(t) is the radio frequency carrier wave.
Of course, c(t-D.sub.base) is the spreading code sequence used in the CDMA system, and would not be present in a non-CDMA system. A pilot signal contains no data, so in the case of a pilot signal d(t)=1 and is constant. The pilot signal spreading code is a different PN code from the data spreading code, allowing the two signals to be distinguished. Once the pilot code timing is known, that same timing can be applied to the data spreading code to allow the receiver to demodulate the digital data.
The process of acquisition, then, is the process of determining the value of D.sub.base. Once the value of D.sub.base is determined, the mobile can use the same c(t-D.sub.base) sequence to lock on to the base signal and remove the spreading code to retrieve the data, d(t).
The quadrature demodulator circuit 16 removes the carrier wave portion, cos(t), from the incoming RF signal and provides a complex valued baseband signal to the sampling circuit 20 which converts the analog RF into digital samples at the spread spectrum frequency of 1.2288 MHZ. At line 22, the signal can be expressed as EQU S.sub.22 =d(t)c(t-D.sub.base)
The base station delay, D.sub.base, is not known by the mobile unit at power up. If D.sub.base is known, then the PN code delay at the mobile unit, D.sub.mobile, can be set to match D.sub.base, and S.sub.22 can be multiplied by c(t-D.sub.mobile) to eliminate the spreading sequence to retrieve the data. Alternatively expressed, if D.sub.mobile =D.sub.base, then EQU d(t)c(t-D.sub.base)c(t-D.sub.mobile)=d(t)c(t-D.sub.base)c(t-D.sub.base) =d(t); because c(t).sup.2 =1